The commercial SpaceX rocket venture launched its unmanned Dragon capsule on a cargo run to the International Space Station on Friday, and then spent hours addressing a gnarly problem with the Dragon's thruster system. The problem was solved, but not before it forced at least a day's delay in the cargo craft's space station rendezvous.

SpaceX's Falcon 9 rocket made a problem-free ascent from its launch pad at Cape Canaveral Air Force Station in Florida at 10:10 a.m. ET to send the Dragon into space. But a half-hour after launch, SpaceX founder Elon Musk said in a Twitter update that controllers encountered an issue involving the capsule's thrusters.

"Issue with Dragon thruster pods," Musk wrote. "System inhibiting three of four [pods] from initializing. About to command inhibit override."

Each pod contains a grouping of thrusters that are used to guide the Dragon's course in orbit. In an email, SpaceX spokeswoman Christina Ra said the Dragon "experienced an issue with a propellant valve" after it achieved orbit. "One thruster pod is running," she said. "We are trying to bring up the remaining three. We did go ahead and get the solar arrays deployed. Once we get at least two pods running, we will begin a series of burns to get to station."

SpaceX's controllers wrestled with the problem for hours. Just before 3 p.m. ET, Musk said that a second thruster pod was up and running. After another hour, he reported that the other two pods were working as well. "Thruster pods one through four are now operating nominally. Preparing to raise orbit. All systems green," Musk said on Twitter. And an hour after that, he sent another tweet saying that the orbit-raising burn was successful. "Dragon back on track," he wrote.

During a teleconference with reporters, Musk speculated that there was a stuck valve or "potentially some blockage" in the lines for pressurizing the thrusters' oxidizer tank. Cycling the valves and releasing a blast of pressurized helium cleared the lines, he said. There was no indication that the blockage did any damage to the system, although SpaceX and NASA were taking a closer look at the cause of the problem and its aftermath.

Musk voiced relief that operations were getting back to normal. "It was a little frightening there," the 41-year-old billionaire coolly acknowledged.

The SpaceX Falcon 9 rocket lifts off from Cape Canaveral Air Force Station in Florida, sending the Dragon capsule on a resupply mission to the International Space Station.

SpaceX

Hundreds of SpaceX employees gather around Dragon mission control at the company's headquarters in Hawthorne, Calif., to watch the Falcon 9 liftoff. A flown Dragon capsule is suspended from the ceiling.

Checking the systemNASA said three operational thruster pods would be required for the Dragon's approach to the space station. The agency's space station manager, Mike Suffredini, said NASA's team would need some "added time to make sure this is working properly." That means the earliest opportunity for astronauts to grab the Dragon with the station's robotic arm and bring it in for a berthing will come early Sunday rather than on Saturday.

NASA's associate administrator for human exploration and operations, Bill Gerstenmaier, praised SpaceX for its handling of the problem. "They did everything exactly right about the vehicle," he said.

This is the third Dragon flight to the station: The first one, which took place last May, was a demonstration flight aimed at proving that California-based SpaceX could safely reach the space station, get hooked up, and then descend again to a splashdown. Last October's second flight marked the first of what's expected to be 12 resupply missions to the station, under the terms of a $1.6 billion contract with NASA. At that rate, each Dragon mission costs NASA about $133 million.

If the Dragon is not able to hook up with the space station, SpaceX would receive only a partial payment for the flight, Musk said. He didn't say how much that payment would amount to.

What's on the Dragon?This Dragon contains more than 2,300 pounds (1,050 kilograms) of cargo, including experiments to study the growth of plants and mouse stem cells in zero-G. There are also spare parts for the station's air-recycling system, and a research freezer for preserving biological samples.

A similar freezer was loaded up with ice cream treats for the crew for last October's resupply mission, but this time, the goodies packed on the Dragon were "a little bit healthier," SpaceX President Gwynne Shotwell said. Although she wasn't specific about what the space station's six residents would be getting, she said the treats were coming fresh from an orchard owned by the father of one of SpaceX's employees.

The astronauts are due to open the Dragon's hatch on the day after its arrival. It will take about three weeks to unload the craft, then load it up with more than 3,000 pounds (1,370 kilograms) of cargo for return to Earth. The original schedule called for the Dragon to be unberthed for a Pacific splashdown and recovery on March 25. Suffredini said that schedule would be adjusted, depending on the time frame for the Dragon's berthing.

SpaceX's cargo flights are meant to fill the gap left by the retirement of NASA's space shuttle fleet in 2011. Another company, Orbital Science Corp., has a separate NASA contract to begin deliveries to the space station later this year. Cargo can also be delivered to the space station on Russian, Japanese and European transports, but only SpaceX currently has the capability to bring cargo back down.

SpaceX and two other companies, Sierra Nevada Corp. and the Boeing Co., are developing crew-capable spacecraft under a separate NASA program. Those spaceships could be ready for NASA's use as early as 2017. In the meantime, U.S. astronauts have to ride on Russian Soyuz capsules at a cost of about $60 million per seat.